In oil production processes, a mixture of oil and water is often recovered. The mixture that is recovered in this way is unwanted and needs to be disposed of. However, it is not environmentally appropriate to dispose of water while it is still contaminated with oil. Therefore, there is a need to separate the oil from the water.
In practice, there are certain limitations on the apparatus that can be used for this separation. For onshore applications, large skim tanks in combination with flotation equipment have been used to remove oil from water. However, in certain urban oil production locations, the use of tanks and non-pressurized separation equipment is under scrutiny due to their emission of hazardous pollutants to the atmosphere as well as their large area footprint. In addition, the constraints of offshore oil production, such as the size of the offshore platform, require that the separation apparatus is both effective and compact. Over recent decades, cyclone separators have been developed to meet these requirements. Specifically, deoiling hydrocyclones for the removal of oil from water have become popular for offshore applications in the oil and gas industry.
A deoiling hydrocyclone separator operates by converting pressure energy into velocity as a fluid mixture of water and oil enters the hydrocyclone through a tangential inlet. This causes the fluid inside the hydrocyclone to spin, which creates a centrifugal force thousands of times higher than the force of gravity within the fluid. The centrifugal force multiplies the natural buoyancy of small oil droplets that have a relatively low density within the water, which has a relatively high density. Consequently, the heavier water phase is directed towards the edges of the hydrocyclone, while the lighter oil phase is retained at the center of the hydrocyclone. The two phases of oil and water can then be extracted from the hydrocyclone separately; the water is extracted via a clean water outlet while the oil is extracted via a waste reject line.
Compared with alternative separation devices, such as skim tanks, a hydrocyclone separator yields a much faster separation process within a smaller area because the active gravitational force in the skim tank is effectively replaced by centrifugal forces in the hydrocyclone, which are of a far higher magnitude. These high centrifugal forces also allow hydrocyclone separators to be relatively insensitive to motion and orientation, making them particularly ideal for offshore applications in the oil industry.
Nevertheless, there remain difficulties in implementing effective hydrocyclone separator systems at a reasonable cost with the required reliability. Existing deoiling hydrocyclone arrangements typically comprise a hydrocyclone separator that receives a mixture of water and oil from an upstream fluid store, and rejects the separated fluids via a clean water outlet and an oily waste outlet. Such systems require a substantial pressure drop between the inlet to the hydrocyclone and the oily waste outlet. Therefore, the upstream fluid store must be operated at a high pressure which can reduce the flow rate of fluids from oil wells feeding the upstream fluid store. The oily waste fluid must also be discharged to a low pressure receiving system.
Furthermore, it is necessary in this existing system to apply a back pressure to the hydrocyclone from the clean water outlet in order to ensure that the oily waste product is forced through the oily waste outlet. This is achieved using a control valve at the clean water outlet, across which a pressure differential is established and which dissipates pressure energy through turbulent friction. This control valve is automated to control the interface between the oil and water phases in the upstream fluid store at a constant level. As the rate of fluid entering the upstream fluid store varies over time, this control valve opens or closes in order to maintain the constant level, which causes a variable flow rate of fluid through the hydrocyclone. This creates a complication in the effective performance of the hydrocyclone system because it is often desirable to achieve a constant flow rate through the hydrocyclone.
Another example of a known hydrocyclone system, which addresses the problem of variable flow rate through the hydrocyclone and the need to operate the upstream fluid store at an elevated pressure, utilises a pump that draws fluid from the upstream fluid store and feeds the fluids to the hydrocyclone together with a plurality of automated control valves to regulate the flow of fluids in the hydrocyclone system. A first control valve is placed between the oily waste outlet of the hydrocyclone and the upstream fluid store and may be opened or closed as desired in order to regulate the flow of oily waste fluid back to the fluid store. This valve may also direct the oily waste to a lower pressure receiving system. A second control valve is placed between the clean water outlet of the hydrocyclone and the upstream fluid store in a similar way to the first control valve. The fluid connection between the clean water outlet and the upstream fluid store may be termed the “recycle line”. A third control valve is also placed at the clean water outlet of the hydrocyclone. This control valve is automated to control the interface between the oil and water phases in the upstream fluid store at a constant level, and regulates the flow of fluid leaving the hydrocyclone separator system according to the interface level in the upstream fluid store.
A problem with this configuration is that, should there arise a situation whereby the hydrocyclone requires to be taken out of service for maintenance or its fluid channels become obstructed, or if its capacity becomes insufficient to maintain a constant level in the upstream fluid store, then a separate bypass line must be added in order to avoid the entire hydrocyclone system being taken offline. However, known bypass lines require manually operated valves in order to bypass fluids around the hydrocyclone system. This introduces a lag time between the identification of an obstruction or capacity constraint and manually adjusting the valves to create the bypass.
There is an ongoing desire to improve fluid separation apparatus for use in onshore and offshore oil operations and elsewhere. In particular, there is a desire to increase the separation efficiency of the hydrocyclone system and maintain a constant flow rate of fluid through the hydrocyclone while simultaneously retaining reliability of the system and avoiding prohibitive expenses.